1. Field of Invention
The present invention generally relates to electrically active ceramic devices and, more particularly, to a method of making high deformation flextensional piezoelectric transducers.
2. Description of the Prior Art
Piezoelectric and electrostrictive materials develop a polarized electric field when placed under stress or strain. Conversely, they undergo dimensional changes in an applied electric field. The dimensional change (i.e., expansion or contraction) of a piezoelectric or electrostrictive material is a function of the applied electric field.
A typical prior ceramic device such as a direct mode actuator makes direct use of a change in the dimensions of the material, when activated, without amplification of the actual displacement. Direct mode actuators typically include a piezoelectric or electrostrictive ceramic plate sandwiched between a pair of electrodes formed on its major surfaces. The device is generally formed of a sufficiently large piezoelectric and/or electrostrictive coefficient to produce the desired strain in the ceramic plate. However, direct mode actuators suffer from the disadvantage of only being able to achieve a very small displacement (strain), which is, at best, only a few tenths of a percent.
Indirect mode actuators are known in the prior art to provide greater displacement than is achievable with direct mode actuators. Indirect mode actuators achieve strain amplification via external structures. An example of an indirect mode actuator is a flextensional transducer. Prior flextensional transducers are composite structures composed of a piezoelectric ceramic element and a metallic shell, stressed plastic, fiberglass, or similar structures. The actuator movement of conventional flextensional devices commonly occurs as a result of expansion in the piezoelectric material which mechanically couples to an amplified contraction of the device in the transverse direction. In operation, they can exhibit several orders of magnitude greater displacement than can be produced by direct mode actuators.
The magnitude of the strain of indirect mode actuators can be increased by constructing them either as "unimorph" or "bimorph" flextensional actuators. A typical unimorph is a concave structure composed of a single piezoelectric element externally bonded to a flexible metal foil, and which results in axial buckling or deflection when electrically energized. Common unimorphs can exhibit a strain of as high as 10% but can only sustain loads which are less than one pound. A conventional bimorph device includes an intermediate flexible metal foil sandwiched between two piezoelectric elements. Electrodes are bonded to each of the major surfaces of the ceramic elements and the metal foil is bonded to the inner two electrodes. Bimorphs exhibit more displacement than comparable unimorphs because under the applied voltage, one ceramic element will contract while the other expands. Bimorphs can exhibit strains up to 20% (i.e. about twice that of unimorphs), but, like unimorphs, typically can only sustain loads which are less than one pound.
A unimorph actuator called "THUNDER", which has improved displacement and load capabilities, has recently been developed and is disclosed in U.S. Pat. No. 5,632,841. THUNDER (which is an acronym for THin layer composite UNimorph ferroelectric Driver and sEnsoR), is a unimorph actuator in which a pre-stress layer is bonded to a thin piezoelectric ceramic wafer at high temperature, and during the cooling down of the composite structure asymmetrically stress biases the ceramic wafer due to the difference in thermal contraction rates of the pre-stress layer and the ceramic layer.
The construction of a prior THUNDER actuator 12 is illustrated in FIG. 1. A PZT piezoelectric ceramic layer 67 which is electroplated 65 and 65a on its two major faces is adhered to a metal pre-stress layer 64 by an adhesive layer 66. The adhesive layer 66 is preferably LaRC-SI.TM. material, a thermoplastic polyimide developed by NASA-Langley Research Center an disclosed in U.S. Pat. No. 5,639,850, and which is commercially marketed by IMITEC, Inc. of Schenectady, N.Y. During manufacture of the THUNDER actuator 12 the ceramic layer 67, the adhesive layer 66 and the pre-stress layer 64 are simultaneously heated to a temperature above the melting point of the adhesive material, and then subsequently allowed to cool, thereby re-solidifying and setting the adhesive layer 66.
In practice the various layers composing the THUNDER actuator (namely the ceramic layer 67, the adhesive layer 66 and the pre-stress layer 64) are typically placed inside of an autoclave or a convection oven as a composite structure, and slowly heated by convection until all the layers of the structure reach a temperature which is above the melting point of the adhesive 66 material but below the Currie temperature of the ceramic layer 67. It is desirable to keep the temperature of the ceramic layer 67 beneath the Currie temperature of the ceramic layer in order to avoid disrupting the piezoelectric characteristics of the ceramic layer 67. Because the multi-layer structure is typically convectively heated at a slow rate, all of the layers tend to be at approximately the same temperature. In any event, because the adhesive layer 66 is typically located between two other layers (i.e. between the ceramic layer 67 and the pre-stress layer 64), the two outer layers (i.e. the ceramic layer 67 and the pre-stress layer 64) are usually very close to the same temperature and are at least as hot as the adhesive layer 67 during the heating step of the process.
During the cooling step of the process (i.e. after the adhesive layer 67 has re-solidified) the ceramic layer 67 becomes compressively stressed by the pre-stress layer 64, due to the higher coefficient of thermal contraction of the material of the pre-stress layer 64 than of the material of the ceramic layer 67. Also, due to the greater thermal contraction of the laminate materials (e.g. the pre-stress layer 64 and the adhesive layer 66) than that of the ceramic layer 67, the ceramic layer typically deforms into an arcuate shape having a normally concave face 12a and a normally convex face 12c, as illustrated in FIG. 1.
In operation the THUNDER actuator 12 may be energized by an electric power supply 22 via a pair of electrical wires 26 and 24 which are in electrical communication with the electroplated 65 and 65a faces of the ceramic layer 67.
It will be appreciated from an understanding of the preceding overview of the prior art that the mechanical output displacement capability of a piezoelectric transducer can be increased by increasing the amount of "pre-stress" applied to the ceramic layer 67 by the pre-stress layer 64.
It will further be understood that the amount of pre-stress which can be applied to a given ceramic layer 67 (having a known coefficient of thermal contraction) by a given pre-stress layer 64 (having a known coefficient of thermal contraction) depends on the change in temperature of the pre-stress layer 64 and depends on the change in temperature of the ceramic layer 67 during the cooling step of the manufacturing process. More specifically, for a given ceramic layer 67 (of known dimensions and known coefficient of thermal contraction) and a given pre-stress layer 64 (of known dimensions and known coefficient of thermal contraction), the amount of pre-stressing of the ceramic layer 67 can be increased by increasing the temperature drop of the pre-stress layer 64 and by decreasing the temperature drop of the ceramic layer 67 during the step of cooling of the adhesive layer 66.
A problem associated with the prior method of manufacturing multi-layered piezoelectric transducers which uses either autoclaves or ovens to heat the pre-stress layer 64, the adhesive layer 66, and the ceramic layer 67 as a composite structure to the same temperature is that (during the adhesive cooling step) the temperature of the ceramic layer 64 drops as far as the temperature of the pre-stress layer 67 drops. Thus, in the prior method of manufacturing such devices, it is not possible to vary (i.e. minimize) the temperature drop of the ceramic layer during the cooling step without disadvantageously reducing the temperature drop of the pre-stress layer 64.
Accordingly, it would be desirable to provide a method of manufacturing a pre-stressed flextensional piezoelectric actuator in which, during the cooling step of the process, the temperature drop of the pre-stress layer 64 is greater than the corresponding temperature drop of the ceramic layer 67.